Diverse Reactivity of Amidinate-Supported Boron Centers with the Hypersilyl Anion and Access to a Monomeric Secondary Boron Hydride.

Diverse reactivity of the bulky tris(trimethylsilyl)silyl substituent [Si(SiMe 3 ) 3 ], also known as the hypersilyl group, was observed for amidinate-supported dichloro- and phenylchloroborane complexes. Treatment of the dichloroborane with potassium tris(trimethylsilyl)silyl led to the activation of the backbone β-carbon center and formation of saturated four-membered heterocyclic chloroboranes R'{Si(SiMe 3 ) 3 }C(NR) 2 BCl [R' = Ph, R = Cy ( 3 ); R' = Ph, R = i Pr ( 6 ); R' = t Bu, R = Cy ( 8 )], whereas the four-membered amidinate hypersilyl-substituted phenyl borane 4 {PhC(NCy) 2 B(Ph)[Si(SiMe 3 ) 3 ]} was observed for the case of an amidinate-supported phenylchloroborane. The highly deshielded 11 B NMR spectroscopic resonance and the distinct difference in the 29 Si NMR spectrum confirmed the presence of a σ-donating hypersilyl effect on compounds 3 , 6 , and 8 . Reaction of 3 with the Lewis acid AlCl 3 led to the formation of complex 11 in which an unusual cleavage of one of the C-N bonds of the amidinate backbone is observed. Nucleophilic substitution at the boron center of saturated chloroborane 3 with phenyllithium generated the phenylborane derivative 12 , whereas the secondary monomeric boron hydride 13 was observed after treatment with alane (AlH 3 ). All compounds ( 2 - 13 ) have been fully characterized by NMR spectroscopy and single-crystal X-ray structure determination studies.